Congenital nystagmus image motion: influence on visual acuity at different luminances

In persons with congenital nystagmus (CN), the ability to integrate visual information over time can be limited by two factors--the duration of foveation periods and the temporal integration period of the visual system. The purpose of this study was to assess the relative importance of these two factors for visual acuity for targets of different luminances. We measured visual acuity using Landolt C targets at 5 luminance levels (50 to 0.005 cd/m2) in 6 observers with CN, and in 6 normal observers with comparable motion of the retinal image. To allow comparison, normal observers viewed the targets during image motion simulating jerk CN, with "foveation durations" ranging from 20 to 160 ms. In the normal observers, acuity improves as a function of the simulated foveation duration at all luminance levels. However, this improvement is larger and occurs at a faster rate at high than at low luminances. The more gradual improvement in acuity with simulated foveation durations at low luminances is consistent with a prolongation of the temporal integration period, which we estimate to range from approximately 140 to 380 ms over the 4 log unit range in luminance that we tested. In observers with CN, the change in acuity with luminance is similar, but not identical, to that in normal observers when the duration of the foveation periods is matched. We conclude that the integration of visual information may be limited by either the period of temporal integration or the duration of the foveation period in persons with CN, depending upon which is shorter at the luminance level under consideration.